Method of making a heat pipe having an easily contaminated internal wetting surface

ABSTRACT

A novel method of making a heat pipe having an easily contaminated internal wetting surface, comprising cleaning the surface and then coating the cleaned surface with a material that is soluble in the working fluid at an operating temperature of the heat pipe. For a heat pipe employing a mercury metal working fluid, easily-oxidized ferrous metal surfaces of the capillary structure and envelope inner wall may be coated with nickel.

United States Patent Freggens [451 June 27, 1972 [54] METHOD OF MAKING A HEAT PIPE HAVING AN EASILY CONTAMINATED INTERNAL WETTING SURFACE [72] Inventor: Robert Alfred Freggens, Lancaster, Pa. [73] Assignee: RCA Corporation I [22] Filed: May 18, 1970 [2]] Appl. No.: 38,323

[52] US. Cl. ..29/157.3 R, 29/424, 29/458,

123/41.4l, 165/105, 165/133 [51] Int. Cl. ..B23p 15/26, B2ld 53/00 [58] Field of Search ..29/157.3 R, 458, 424;

[56] I References Cited UNITED STATES PATENTS 1,663,709 3/1928 Kettering ..165/105 X 1,748,518 2/1930 Midgley ..165/ 105 X Primary Examiner-Charlie T. Moon Assistant Examiner-Donald C. Reiley, III Attorney-Glenn H. Bruestle [57] ABSTRACT A novel method of making a heat pipe having an easily contaminated internal wetting surface, comprising cleaning the surface and then coating the cleaned surface with a material that is soluble in the working fluid at an operating temperature of the heat pipe. Fora heat pipe employing a mercury metal working fluid, easily-oxidized ferrous metal surfaces of the capillary structure and envelope inner wall may be coated with nickel.

10 Claims, 2 Drawing Figures METHOD OF MAKING A HEAT PIPE HAVING AN EASILY CONTAMINATED INTERNAL WETTING SURFACE BACKGROUND OF THE INVENTION This invention relates to a novel method of making a heat pipe having an easily contaminated internal wetting surface.

A type of heat pipe having an easily contaminated internal wetting surface is a heat pipe employing a mercury metal working fluid, a stainless steel capillary structure, and a stainless steel envelope inner wall. Heat pipes of this type operate unsatisfactorily because the stainless steel surfaces tend to become. oxidized during heat pipe construction. Oxidized or un-clean" surfaces are poorly wetted by the working fluid, reducing the capillary pumping pressure from that obtainable with clean" surfaces.

Previous heat pipes have employed scavengers" to remove the oxides from the stainless from stainless steel surface. For example, active metals such as calcium, magnesium, and sodium have been added to the mercury metal working fluid prior to evacuating the heat pipe. However, this method results in the formation of compounds, such as Na O,-Fe,O which are insoluble in the working fluid. These compounds, in turn, tend to block or plug up the small pores of the capillary structure, whereby the capillary pumping is again inhibited. Also, the operating life of the heat pipe may be reduced by the erosion of the envelope by the working fluid during heat pipe operation.

SUMMARY OF THE INVENTION The novel method applies to making a heat pipe comprising a sealed'envelope containing at least one surface that is wettable by a working fluid, the surface being easily contaminated so as to be poorly wetted by the working fluid. The novel method comprises cleaning the surface to remove contaminents therefrom and then coating the cleaned surface with a material that is soluble in the working fluid at an operating temperature of the heat pipe. The heat pipe is then assembled, evacuated, and sealed.

By coating the cleaned surface, it is protected against contamination, particularly by oxidation. By using a .coating material that is soluble in the working fluid, the protective coating can be removed by the-working fluid after the heat pipe is sealed. Scavengers may be omitted from the working fluid, thus obviating the formation of undesirable compounds in the heat pipe. Heat pipes made by the novel method operate longer and more efficiently than do heat pipes made by previous methods. Also, coatingmaterial dissolved in the working fluid reduces the erosive action of the working fluid on the envelope during operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a heat pipe made by the novel method; and

FIG. 2 is a sectional view of an end portion of the heat pipe of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following is an example of the novel method. A heat pipe, as shown in FIG. 1, comprises an envelope 11 consisting of a long, thin tubular member 13 made of stainless steel. The tubular member 13 is sealed at one end to a stainless steel end cap 15 and at the other end to a stainless steel exhaust cap 17 having an exhaust tubulation l9. Disposed within the envelope 11, and adjacent an inner wall 21 thereof, is a cylindrical mesh screen or capillary structure 23 made of stainless steel. The envelope 11 also contains a mercury metal working fluid in contact with and saturating the capillary structure 23.

Prior to assembling the heat pipe, the envelope parts l3, l5, and I7 and the capillary structure 23 are each cleaned chemiproduce thin protective coatings thereon. As shown in FIG. 2, a coating 25 is produced on each of the inner walls of the envelope 11, and a coating 27 is produced on the surface of the capillary structure 23. The envelope parts l3, l5, and 17 and the capillary structure 23 are then re-fired at a lower temperature, about 800 C., to remove contaminents which may have entered the clean stainless steel surfaces or been codeposited during the plating process.

' Following the above steps, the capillary structure 23 is inserted within the tubular member 13, and the end cap 15 is electron-beam welded to one end of the tubular member 13. The mercury metal working fluid is then added, under an inert atmosphere of argon, to the end-capped tubular member 13 at the opposite, open end thereof. The open end of the tubular member 13 is, in turn, electron-beam welded to the exhaust cap 17, the exhaust tubulation 19 of which is connected to a suitable vacuum exhaust system (not shown). It should be noted that through this step, the nickel coatings 25 and 27 are insoluble in the mercury metal working fluid. The assembled heat pipe is then evacuated at an elevated temperature, about 400 C., to further clean the system. Since the operating temperature range of the heat pipe is between about 250 C. and about 450 C., portions of the nickel coatings 25 and 27 are dissolved by the mercury metal working fluid during the evacuating step. During this step, mercury vapor is contained in the envelope 11 by heating the exhaust tubulation to about 800 C. Upon cooling, the exhaust tubulation 19 is tipped off to produce a sealed, vacuum-tight envelope 1 1.

After it is sealed, the heat pipe may be operated by suitably heating either end region thereof to a temperature within the range between about 250 C. and about 450 C. The region adjacent the end cap l5-tubular member 13 seal is usually chosen as the heat input or evaporator region of the heat pipe; and the region adjacent the exhaust cap l7-tubular member 13 seal is usually the heat output or condenser regionof the heat pipe. In the evaporator region of the heat pipe, the input heat thereto is absorbed by the mercury metal working fluid, which is then evaporated from the capillary structure 23. The vapor thus formed enters the hollow volume of the heat pipe and flows to the cooler, condenser region of the heat pipe. In the condenser region, the vapor is condensed, and the condensate thus formed is absorbed by the capillary structure 23. The capillary structure 23 then "pumps" the condensate back to the evaporator region, and the cycle is repeated.

For eflicient operation of the heat pipe, the pumping capability of the capillary structure 23 should be as great as possically and fired in a reducing atmosphere at about 900 C. to i ble. That is, the capillary pumping pressure should be as high as possible. The capillary pumping pressure is, in turn, related to the extent to which the capillary structure 23 is wetted by the mercury metal working fluid. At the start of the heat pipe operation as described above, the capillary pumping pressure is relatively low, because the nickel-plated capillary structure is not well wetted by the working fluid. It will be recalled that the nickel coatings 25 and 27 are only partially dissolved by the mercury metal working fluid during the evacuating step. However, as the operation of the heat pipe proceeds, more and more of the coatings 25 and 27 are dissolved by the working fluid, the wetting of the capillary structure 23 by the working fluid improves, whereby the capillary pumping pressure increases. When all of the coatings 25 and 27 are dissolved by the working fluid, maximum wetting occurs between the clean stainless steel surfaces of the capillary structure 23 and the mercury metal working fluid, in which are dissolved the coatings 25 and 27.

Preferably, the coating 25 and 27 are thick enough, that is, the amount of coating material is sufiicient, to saturate the mercury metal working fluid with the dissolved nickel. Where the amount of coating material is insufficient to saturate the with some nickel, thereby limiting the capillary pumping pressure in the heat pipe.

A typical heat pipe made by the novel method described above has an overall length of 14.27 inches, an outer diameter of 0.438 inch, and a wall thickness of 0.020 inch. The envelope 11 is made of type 304 stainless steel, and the capillary structure 23 comprises 2.25 wraps of 60x60 stainless steel mesh screen. The nickel coatings 25 and 27 each have an initial thickness of about 0.0002 inch. A heat pipe as described above was operated at a temperature of about 325 C., with a thermal transfer of about 340 watts, for greater than 25,000 hours. The performance of the heat pipe was studied in terms of the wetting angle between the working fluid and the capillary structure 23, as calculated from the equation P being the capillary pumping pressure, r being the pore radius of the capillary structure, ,8 being the surface tension of the working fluid, and 1: being the wetting angle between the working fluid and the capillary structure. After 50 hours of operation, d; was about 72; after 400 hours, was about 63; and after 4,500 hours, (b was about 57. Since it had been shown elsewhere (see, for example, J. J. Droher, Studies of lnterfacial Effects between Mercury and Steel, ORO-69, Atomic Energy Commission, University of Tennessee, June 1952) that wetting angles as small as 57 could be obtained between mercury and clean" stainless steel, the heat pipe results confirmed the description of the operation given above. With prior heat pipes, the wetting angle d1 often exceeded 90 and was as large as 140 thus causing the capillary pumping to cease.

GENERAL CONSIDERATIONS It should be understood that the invention is not limited to the description given above. For example, the heat pipes may employ re-entrant, annular, angular, and flat-plate designs as well as the cylindrical design shown in FIG. 1. Several such geometries are illustrated in an article by G. Y. Eastman, enti tled The Heat Pipe, appearing in Scientific American, 218, 38-46 (1968). The capillary structures may also be altematively designed; and they may be spaced from, rather than adjacent to, the envelope walls so as to form arterial return paths for the working fluids in the heat pipes. A heat pipe having a capillary structure spaced from an envelope wall is illustrated in US. Pat. No. 3,435,889.

Various combinations of envelope, capillary structure, and working fluid materials may be employed. For mercury metal working fluids, the wettable surfaces may be made of ferrous metals other than stainless steel, such as iron and nickel-iron alloys. The envelopes and capillary structures may be made of different materials. Capillary pumping may be effected by porous-fiber and grooved capillary structures. The heat pipes may also employ vapor ducts for separating the working fluid vapors from the returning condensates in the condenser regions thereof. Such vapor ducts are described in US. Pat. application Ser. No. 640,693, filed on May 23, 1967, by W. E. Harbaugh, now Pat. No. 3,568,762. The protective coatings may be made of iron as well as nickel, and may be plated or otherwise deposited onto the wettable surfaces.

Also, the cleaning and exhaust procedures may be modified with respect to those described above. For example, the initial firing of the envelope and capillary parts may be performed in a high-vacuum atmosphere at l,000 C. or higher. And, the refiring of the coated parts may be eliminated where the procedure is found to be unnecessary.

I claim:

1. A method of making a heat pipe comprising a sealed envelope containing a capillary structure, including at least one surface in said heat pipe being wettable by a mercury metal working fluid, said surface being easily contaminated so as to be poorly wetted by said mercury metal working fluid, comprising the steps of:

a. Cleaning said surface to remove contaminents therefrom;

b. coating said cleaned surface with a thin layer of a material that is soluble in said working fluid at an operating temperature of said heat pipe in order to protect said cleaned surface from contamination and to reduce the erosive action of said mercury metal working fluid during operation;

0. assembling said heat pipe;

d. adding said mercury metal working fluid to said assembled heat pipe;

e. evacuating said assembled heat pipe; and

f. sealing said evacuated heat pipe.

2. The method of claim 1, wherein said capillary structure comprises said surface.

3. The method of claim 1, wherein the inner wall of said envelope comprises said surface.

4. A method of making a heat pipe comprising a sealed envelope containing a capillary structure, including at least one ferrous metal surface in said heat pipe being wettable by a mercury metal working fluid, comprising the steps of:

a. cleaning said surface to remove oxides therefrom;

b. coating said cleaned surface with a thin layer of a metal soluble in said working fluid at an operating temperature of said heat pipe in order to protect said cleaned surface from contamination and to reduce the erosive action or said mercury metal working fluid during operation;

c. assembling said heat pipe;

d. adding said mercury metal working fluid to said assembled heat pipe;

e. evacuating said assembled heat pipe; and

f. sealing said evacuated heat pipe.

5. The method of claim 4, wherein said capillary structure is of stainless steel and comprises said surface.

6. The method of claim 4, wherein said soluble metal is nickel.

7. The method of claim 6, wherein said nickel coating is applied by plating prior to said assembling step.

8. A method of making a heat pipe comprising a sealed envelope containing an inner wall and a capillary structure each having a ferrous metal surface that is wettable by a mercury metal working fluid, comprising the steps of:

a. cleaning each of said surfaces to remove oxides therefrom;

b. coating each of said cleaned surfaces with a thin layer of a metal that is soluble in said working fluid at an operating temperature of said heat pipe in order to protect said cleaned surface from contamination and to reduce the erosive action of said mercury metal working fluid during operation;

0. assembling said heat pipe;

d. adding said mercury metal working fluid to said'assembled heat pipe;

e. evacuating said assembled heat pipe; and

f. sealing said evacuated heat pipe.

9. The method of claim 8, wherein said soluble metal is nickel.

10. The method of claim 9, wherein the amount of said soluble metal in said coatings is sufficient to saturate said working fluid therewith.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,672,020 Dated June 1972 Ifiven m-(S) Robert Alfred Freggens It is certified that error appears in the above-identified gatent and that said Letters Patent are hereby corrected as shown below:

Column 1, lime 19, delete "from stainless steel surface" should read steel L surfaces- Column 3, line 17,

"B" Should read --OL--. Column 4, line 31, "or" should read -of-.

Signed and sealed this 26th day of September 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents F ORM PO-1050 (10-69) USCOMM'DC 60376-969 3530 6|72 s u s sovnuucm rnmvma ornc: 1909 0-in-3): 

2. The method of claim 1, wherein said capillary structure comprises said surface.
 3. The method of claim 1, wherein the inner wall of said envelope comprises said surface.
 4. A method of making a heat pipe comprising a sealed envelope containing a capillary structure, including at least one ferrous metal surface in said heat pipe being wettable by a mercury metal working fluid, comprising the steps of: a. cleaning said surface to remove oxides therefrom; b. coating said cleaned surface with a thin layer of a metal soluble in said working fluid at an operating temperature of said heat pipe in order to protect said cleaned surface from contamination and to reduce the erosive action or said mercury metal working fluid during operation; c. assembling said heat pipe; d. adding said mercury metal working fluid to said assembled heat pipe; e. evacuating said assembled heat pipe; and f. sealing said evacuated heat pipe.
 5. The method of claim 4, wherein said capillary structure is of stainless steel and comprises said surface.
 6. The method of claim 4, wherein said soluble metal is nickel.
 7. The method of claim 6, wherein said nickel coating is applied by plating prior to said assembling step.
 8. A method of making a heat pipe comprising a sealed envelope containing an inner wall and a capillary structure each having a ferrous metal surface that is wettable by a mercury metal working fluid, comprising the steps of: a. cleaning each of said surfaces to remove oxides therefrom; b. coating each of said cleaned surfaces with a thin layer of a metal that is soluble in said working fluid at an operating temperature of said heat pipe in order to protect said cleaned surface from contamination and to reduce The erosive action of said mercury metal working fluid during operation; c. assembling said heat pipe; d. adding said mercury metal working fluid to said assembled heat pipe; e. evacuating said assembled heat pipe; and f. sealing said evacuated heat pipe.
 9. The method of claim 8, wherein said soluble metal is nickel.
 10. The method of claim 9, wherein the amount of said soluble metal in said coatings is sufficient to saturate said working fluid therewith. 